The circles and intersect in two distinct points and . The tangents to through and intersect at . Let be an arbitrary point on the circle distinct from and . The line meets the circle again at , the line meets the circle again at and the line meets the circle again at . Prove that the line contains the midpoint of the segment .
Solution
Let be the intersection of the lines and .
By Menelaus' theorem for the triangle and the colinear points and we have
In order to show that it suffices to see that
(Note: We will be using directed angles from this point on.)
The points and are concyclic, so . The points and are also concyclic, so . The triangles and have two angles in common, so they are similar. It follows that
By the tangent-chord angle theorem we have . The triangles and have two angles in common and are similar. Using the tangent-chord angle theorem again we see that . So, the triangles and have two angles in common and are similar. From here we find the ratios and . Combining the two with the equality between the tangent segments we get
(Note: The property (14) or is called the harmonic property of the cyclic quadrilateral.)
If we now combine the equalities (13) and (14), we get (12), which was our goal. From here we conclude that is the midpoint of and so the midpoint of does indeed lie on .
